import java.util.*;

/**
 * Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.
 * <p>
 * Note: The solution set must not contain duplicate quadruplets.
 * <p>
 * For example, given array S = [1, 0, -1, 0, -2, 2], and target = 0.
 * A solution set is:
 * [
 *  [-1,  0, 0, 1],
 *  [-2, -1, 1, 2],
 *  [-2,  0, 0, 2]
 * ]
 * <p>
 * Created by drfish on 6/20/2017.
 */
public class _0184Sum {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> result = new ArrayList<>();
        if (nums.length < 4) {
            return result;
        }
        Arrays.sort(nums);
        for (int i = 0; i < nums.length - 3; i++) {
            if (nums[i] + nums[i + 1] + nums[i + 2] + nums[i + 3] > target) {
                break;
            }
            if (nums[i] + nums[nums.length - 1] + nums[nums.length - 2] + nums[nums.length - 3] < target) {
                continue;
            }
            if (i > 0 && nums[i] == nums[i - 1]) {
                continue;
            }

            for (int j = i + 1; j < nums.length - 2; j++) {
                if (nums[i] + nums[j] + nums[j + 1] + nums[j + 2] > target) {
                    break;
                }
                if (nums[i] + nums[j] + nums[nums.length - 1] + nums[nums.length - 2] < target) {
                    continue;
                }
                if (j > i + 1 && nums[j] == nums[j - 1]) {
                    continue;
                }

                int low = j + 1, high = nums.length - 1;
                while (low < high) {
                    int sum = nums[i] + nums[j] + nums[low] + nums[high];
                    if (sum == target) {
                        result.add(Arrays.asList(nums[i], nums[j], nums[low], nums[high]));
                        while (low < high && nums[low] == nums[low + 1]) {
                            low++;
                        }
                        while (low < high && nums[high] == nums[high - 1]) {
                            high--;
                        }
                        low++;
                        high--;
                    } else if (sum < target) {
                        low++;
                    } else {
                        high--;
                    }
                }
            }
        }
        return result;
    }
}
